1. Field of the Invention
This invention relates to medical apparatus and more particularly to an implantable lead for electrical stimulation of selected portions of the body of the animal in which the lead is implanted and in particular the spinal cord thereof. The word "animal" is here used in its broad sense, including Homo Sapiens.
2. State of the Prior Art
It has long been known to transmit and apply electrical signals to and from the body by way of electrical contact, either with the skin or with respect to a particular organ therein. For example, electrical stimulation has been applied to the heart to stimulate the heart into contracting and expanding at a prescribed, selected rate, as described in U.S. Pat. No. 3,057,356 of W. Greatbatch and assigned to the assignee of the subject invention. In addition, it has been known to apply an electrical signal to the skin to accomplish an electrical "treatment" as discussed in U.S. Pat. Nos. 1,059,090 and 1,305,725. As medical science has advanced, it has been increasingly realized that electrical signals may be applied to relieve or at least block the sensation of pain and in particular chronic, intractable pain. Chronic, intractable pain, by definition implies that all conservative medical treatments aimed at the pain-causing lesion have been instituted and performed respectively. When such procedures fail, the physician and in particular the neurosurgeon, is faced with the finality of destroying a portion of the nervous system tissue in an attempt to sever the pathways of the pain stimulus to the brain, or, in severe cases, to remove portions of the brain tissue itself. For decades, research has been directed at developing more desirable neurosurgical approaches to consistent ablation of chronic, intractable pain. To this end, it has been discovered that electrical stimulation may be selectively applied to the dorsal column or spinal column of the patient, whereby controlled interruption of pain transmission provides a non-destructive approach to the management of pain and care for selected patients.
It has been demonstrated physiologically that all pain information travels through the spinal cord via gamma-delta or C-fibers. Also, stimulation of the larger A-beta fibers is never painful. It has been hypothesized that activity in beta-fibers inhibits, at the first spinal synapse, immediate subsequent activity from the smallest fibers considered essential to pain conduction. As discussed in the publication, "Pain Mechanisms: A New Theory", by Melzack and Wall, Science, 1965, Vol. 150, pp. 971-973, this mechanism may act to balance pain and non-pain input. It has been experimentally demonstrated, as reported by C. N. Shealy et al in "Electrical Inhibition of Pain: Experimental Evaluation", Anesth. Analg. Curr. Res., 1967, 46:299, that the pain threshold may be increased by 12-fold during dorsal column stimulation. This technique has subsequently been applied to human patients with chronic intractable pain. In response thereto, patients sense paresthesiae, i.e. a buzzing or tingling sensation, that radiates downward through much of the body below the implanted electrode.
To relieve pain through dorsal column stimulation, the electrode is implanted and connected by a lead to a receiver which also may be implanted within the patient's body. Typically, a transmitter is provided externally of the patient for exciting the implanted transmitter, whereby an electrical signal of selected frequency, amplitude and duration, is applied to the electrode inserted within the dorsal cord. Clinical results indicate that efficient pain relief occurs only in those areas where the paresthesiae is sensed. Thus, as will be discussed in some detail later, it is significant to have patient response as to the paresthesiae and his pain relief in order to ensure the maximum efficient use of dorsal column stimulation. It has been shown that the use of electrical stimulation has many advantages over the use of drugs or destructive surgery wherein a portion of the peripheral or central nervous system tissue is destroyed, to relieve intractable pain. Drugs invariably have the disadvantage of undesirable physiological side effects as well as mental obtundation or addition, or both. Destructive surgery (primarily cordotomy or rhizotomy) carries substantial risk of weakness, numbness, dysesthesia, bladder or bowel incontinence, impotentia and irreversibility.
Typically, such dorsal column stimulator systems contain a radio frequency transmitter, a transmitting antenna, both of which are positioned externally of the patient's body, and an RF receiver and a stimulating lead implanted, as will be explained, within the patient's body. The transmitter supplies stimulating power of a selected frequency, amplitude and pulsewidth to the receiver by inductive coupling through the patient's skin. In operation, the antenna is placed on the skin directly over the implanted receiver, whereby an inductive coupling is established therebetween. The receiver shapes the RF signals into appropriate waveform and applies this electrical energy via the implanted leads to the dorsal column.
In the prior art, the leads have been implanted, typically by a procedure known as a "bilateral laminectomy" wherein a skin incision is made of sufficient length to expose the dura mater to implant a pair of electrodes to stimulate the afferent nerve disposed within the spinal cord, whereby the transmission of pain therethrough is essentially blocked. A laminectomy is considered to be major surgery in that two or three spinous processes and one set of lamina are removed, with resultant trauma upon the patient. In addition, a transverse incision is made, centered over the clavical, for the placement of the receiver, and a subcutaneous tunnel is developed from this incision over the shoulder to the laminectomy site.
In any surgical procedure for the implantation of pain-relieving leads, whether by a laminectomy or by another procedure to be described, it is desirable to have a percutaneous screening or testing period, wherein the physician and/or the patient may test the reaction of the patient to the stimulator to block pain. During this screening period, the patient may well have an adverse reaction in terms of pain or discomfort due to the implantation or, for some reason, the electrical stimulation may not have the desired effect. In carrying out the above-discussed laminectomy, it is desirable because of the major nature of the surgery carried out, that the patient be completely anesthetized. As a result, it is not possible for the physician to use a local anesthetic, whereby the patient's reaction to the stimulation may be tested. Otherwise, it would be possible if the patient could be mildly anesthetized in the region of the implantation, to adjust the position of the electrodes with respect to the afferent nerves, to ensure maximum pain relief sensation at minimum levels of excitation in terms of amplitude and/or pulsewidth, noting that as these parameters increase, battery life and possible adverse effects are respectively reduced and increased. Before a typical laminectomy procedure, the neurosurgeon places a pair of temporary leads within the spinal canal by a percutaneous procedure. If the electrodes of these leads are accurately placed and the patient is appropriately responsive to the electrical stimulation, the electrode site within the patient may be exposed by laminectomy and the temporary leads replaced with permanent leads; the difficulty with such a replacement is that the electrode position may well be changed with the result that the subsequent positioning of the electrodes of the permanent leads with respect to the afferent nerves is not as beneficial.
The significance of the percutaneous screening resides in the opportunity to obtain the patient's reaction to such electrical stimulation and to the implantation of a foreign body within his body. For example, the patient may not be psychologically conditioned to receive such an implant in that he may be dependent upon such pain to obtain attention from friends and relatives or may be monetarily dependent thereon to receive financial aid. In addition, the potential success of such electrical stimulation to reduce pain may be lessened by the previous history of using medication, including narcotic drugs, for pain alleviation. In either case, a patient may not be a suitable candidate for implantation and it would be desirable to test his reaction to such stimulation during a testing or screening period.
Due to the possible adverse effects of a laminectomy, other less strenuous surgical procedures have been developed wherein the patient may be subjected to a local anesthesia in the vicinity of the electrode implantation site and the receiver site. Typically, as described in a manuscript entitled, "Percutaneous System for Epidural Stimulation", as published by the Avery Laboratories, Inc. of Farmingdale, N.Y., the patient is prepared in a prone position with local anesthesia, and a hypodermic needle having a first electrode lead disposed therein is inserted under radiographic control between two vertebrae, several spaces below the desired level of stimulation. The opening of the needle tip should be directed cephalad and should ultimately be within the epidural space. Next, a second electrode lead is inserted by a second hypodermic needle, one space below the first, under radiographic control so that the opening of the needle is within the epidural space and facing cephalad on the midline of the spinal cord. At this point, the tips of the two inserted electrodes are disposed within the epidural space and should lie from 1 to 5 cm above or below each other, as close to the midline as possible. Next, an incision is made through the subcutaneous tissue between the two needle punctures that serves as a site for the electrode plant, and then each hypodermic needle is withdrawn over the electrode lead wire. A further incision to form a connector pocket is made approximately 4 cm lateral to the first incision, and a tunnel is made therebetween through which the first and second leads associated with the aforementioned electrode tips, are passed. At this point, the remote contacts of the leads are temporarily connected to a stimulating generator and suitable stimulation is applied to the leads, whereby the placement of the electrodes within the spinal cord may be slightly adjusted to achieve maximum stimulation. The incisions are closed, with the temporary leads passing externally through the skin to be connected to the test stimulating generator. If the percutaneous screening proves successful, it is then desired to implant the receiver within the receiver pocket. To this end, the connector pocket is opened so as to not damage the electrode leads or the temporary connectors, and these are subsequently disengaged. Next, the temporary connectors are withdrawn and the receiver pocket is formed to receive the receiver and a new tunnel is formed between the receiver site and the connector pocket. Subsequently, the receiver is inserted and its leads are passed through the new tunnel to the connector pocket and subsequently interconnected with the electrode leads. At this time, all the incisions are closed. Thus, if it were desired to conduct percutaneous screening, it is necessary in utilizing such a procedure to form three incisions within the patient. Further, there is the possibility in disconnecting the temporary leads from the electrode leads that it would be necessary to withdraw the ends of both to effect the disconnection; in carrying out this step, it is a distinct possibility that the electrode placement near the dorsal cord may be disturbed, thus negating the original accurate placement by the neurosurgeon. In addition, it is necessary to make an extra incision within the patient before it is decided whether he is adaptive to receive such chronic implantation.
Reference is now made to FIG. 1, marked "Prior Art", which shows an electrode assembly as may be well used in a procedure as described above. Though only a single lead is shown in FIG. 1, it is understood that two such leads would be inserted. In FIG. 1, there is shown a lead 10 comprising at its distal end, an electrode 14 made of a suitable electrically conductive metal capable of being implanted within the body without corrosion, such as platinum. A flexible conductor 16 extends from the electrode 14 to a rigid, typically metallic male connector pin 18. Further, a sheath 12 of a suitable insulating material adapted to be implanted within the human body, is disposed about the conductor 16, which may illustratively be a helically-wound wire, such as a platinum-iridium alloy. The helically-wound conductor 16 is connected at its proximal end to the pin 18 and extends along the length of the lead 10 to be connected to the electrode 14. In addition, a stainless steel stylet 22 may be disposed within a lumen or hollow passage 24 of the connector pin 18 and conductor 16, to assist the surgeon in guiding the lead 10 into its correct position. As generally illustrated in FIG. 1, the stylet 22 extends through an opening within the pin 18 into the lumen 24, and into and along substantially the entire length of the lead 10.
As described above, the surgical procedure for inserting such a lead 10 requires that a second, temporary lead be connected thereto, to be withdrawn through a third incision within the patient before the test period is begun. In addition, such a lead does not facilitate the ready interconnection to the receiver to be implanted with the result that the placement of its electrode 14 may be slightly disturbed during the permanent implantation of the receiver, so that a less than maximum stimulation of the patient is subsequently achieved.